After an oil drilling rig drills a well and installs the well casing, the rig is dismantled and removed from the site. From that point on, a mobile repair unit, or workover rig, is typically used to service the well. Servicing includes, for example, installing and removing inner tubing strings, sucker rods, and pumps. This is generally done with a cable hoist system that includes a traveling block that raises and lowers the aforementioned tubing strings, sucker rods, and pumps.
U.S. Pat. No. 4,334,217 describes a system for monitoring the movement of a traveling block on a drilling rig. As described in the '217 patent, the traveling block can be raised or lowered beyond a safe limit. This is called “crown out” if the traveling block reaches its upper-most safe position, and “floor out” if it reaches its lower-most safe position. Crown out/floor out can result in equipment damage and present a hazard to personnel working on the equipment. Because it is often not possible for the operator of the cable hoist system to see the position of the traveling block, or because the operator can be otherwise distracted from monitoring the position of the traveling block, the operator can inadvertently exceed safe positions of the traveling block.
Drilling rigs and mobile well servicing units alike are often equipped with safety devices that prevent or a least reduce the possibility of the traveling blocks from reaching a crown out or floor out position. The techniques to prevent crown outs and floor outs can vary. These techniques include use of wire trip sensors, radar, ultrasonic sensors located near the crown, drum encoders, and wire rope counters as sensing devices that determine the blocks are too close to the crown. In addition, the safety systems may incorporate programmable logic control (“PLC”) circuits to detect and actuate cylinders. In another embodiment, the safety system can be configured in a “fail safe” mode. When a circuit is broken in the fail safe mode, a cylinder is activated, thereby activating the braking system on the hoist to prevent the drilling line from additional movement.
While many different methods exist for preventing crown outs and floor outs, most have a single commonality, that being when a sensor or other device detects the pending crown out or floor out, the device sets the brakes for the tubing drum via a pneumatic or hydraulic cylinder. FIG. 1 provides a detailed drawing of a conventional tubing drum brake assembly 100. Although only one side of the drum 105 is shown for the sake of simplicity, a person of ordinary skill in the art will recognize that, in actuality, the tubing drum 105 typically has two brake flanges 115, one on each side of the tubing drum 105. Each side of the tubing drum 105 includes a brake band 120 wrapped around a flange 115. Each of the brake bands 120 are actuated by a equalizer bar 130. In one conventional embodiment, a common equalizer bar 130 actuates both of the brake bands 120 on the tubing drum 105. As shown in FIG. 1, the brake bands 120 are actuated by the equalizer bar 130 rotating in the clockwise direction, which generates tension on the brake bands 120, causing them to tighten up and apply pressure to the brake flanges 115. The pressure applied by the brake bands 120 causes the tubing drum 105 to slow or stop its rotation about the tubing core 110 depending on the amount of pressure applied by the brake bands 120 and causing a corresponding reduction in movement of the feed line 160 as shown in FIGS. 1-4.
As further shown in FIG. 1, the conventional braking system can rotate the equalizing bar 130, by applying a downward force on a brake handle 155. The brake handle 155 is attached to a brake lever 150 which rotates about a pivot point 145 when the downward force is applied to the brake handle 155. The rotation of the brake lever 150 rotates a bell crank 140 in a clockwise direction, the bell crank 140 and brake lever 150 being mechanically coupled to one another. The bell crank 140 is attached at one end to the brake lever 150 and attached at another end to the brake linkage 165 and rotates about pivot point 145. The brake linkage 165 is attached at the other end to the equalizer bar 130. The rotation of the bell crank 140 creates a tension in the brake linkage 165, thereby causing the equalizer bar 130 to rotate in the clockwise direction about a pivot point 125.
As the rig trips into the hole with heavy loads, the braking system heats up and the brake bands 120 expand, which can cause slack in the system 100. To compensate for this problem, conventional braking systems, like the one shown in FIG. 1, include an adjustment swivel 135 as part of the brake linkage 165. The adjustment swivel 135 can be rotated about the braking linkage 165 to shorten or lengthen the linkage 165 as the need arises. For example, as the brake bands 120 retain more heat and get longer, an operator will shorten the length of the brake linkage 165 through the use of the adjustment swivel 135. Conversely, as the brake bands 130 begin to cool and shorten, the operator will lengthen the linkage 165 by rotating the adjustment swivel 135. The objective for the operator is to maintain the brake handle 155 at a height that is comfortable for his use.
In order to effect the safe braking of the rig or hoist, the braking system 100 must operate under tight tolerances, which necessitates that the brake bands 120 remain in constant or virtually constant contact with the brake flanges 115. Because the weight of the brake handle 155 and brake lever 150 will provide sufficient down force to slow the rotation of the drum 105 the operator typically will lift up on the brake handle 155 and lever 150 as shown in FIGS. 2 and 3. By lifting the brake handle 155 and lever 150, the brake bands 120 provide less pressure on the brake flanges 115, thereby allowing the drum 105 to rotate at a higher rate of speed.
As discussed above and shown in greater detail in FIGS. 4 and 5, most conventional rig braking systems 400, 500 include additional features 405 to prevent a crown out or floor out event. The features can include a cylinder 410 which can be pneumatically or hydraulically operated. When activated, the cylinder 410 suddenly extends a rod 415 outward and provides pressure on a lever 420 that is attached to the equalizer bar 130. In response to the pressure, the equalizer bar 130 rotates clockwise causing the brake bands 120 to apply pressure to the brake flange 115, thereby slowing the drum 105 and the feed line 160. However, as shown in FIG. 5, because the equalizer bar 130 is also mechanically linked to the brake handle 155 via the brake linkage 165, bell crank 140, and brake lever 150, the brake handle 155 suddenly moves in the downward direction in response to the cylinder 410 firing. If the rig operator is in the vicinity of, or is holding the brake handle 155 up to reduce braking pressure on the drum 105, when the cylinder 410 fires, the operator may be injured by the brake handle's 155 sudden downward motion. Therefore, there is a need in the art for a braking system that allows the actuating cylinder to act independently of the brake handle so that when the brakes are set due to the detection of a crown out, floor out or other action or problem the brake handle will not be jerked out of the operator's hand and potentially cause injury.